运用超声疲劳试验系统和MTS液压伺服疲劳试验系统开展了Ti-6Al-4V钛合金在不同加载频率下的超高周疲劳试验,使用扫描电镜观察了试样断口形貌并对超高周疲劳试验数据进行了可靠性疲劳寿命分析.结果表明:疲劳循环周数超过107以后,Ti-6Al-4V钛合金试样依然会发生疲劳断裂,不存在传统意义上的疲劳极限.裂纹萌生于试样表面和次表面,随着应力水平降低,萌生位置由表面向次表面转变;试样断口平坦,裂纹源呈现类“鱼眼”特征,裂纹萌生于材料次表面微观组织不均匀处,周围出现黑色颗粒海绵状特征;20 Hz和20 kHz两种频率下的试验数据吻合良好,加载频率对材料的疲劳性能没有明显影响;Ti-6Al-4V钛合金材料的超高周疲劳寿命服从3参数威布尔分布,采用单侧容限系数法建立了不同置信度和不同存活率下的疲劳寿命模型,给出了p-γ-S-N曲线.
The very-high-cycle fatigue test and fatigue reliability life for Ti-6Al-4V were investigated based on the self-built ultrasonic fatigue test system and MTS fatigue system at both 20 Hz and 20 kHz.Fracture morphology of the specimen was analyzed with scanning electron microscopy.The distribution of fatigue life was determined.Single side allowance factor was proposed to calculate the reliability fatigue life with both reliability and confidence level.The results indicate that fatigue life continuously increases with the decrease of the stress amplitude and there is no fatigue limit in the very high cycle regime.The specimens fracture initiates from defects at specimen surface at short fatigue life region and the location of crack initiation tends to subsurface with the fatigue life increases.The specimen fracture surface tested at 20 kHz is smoother than that of 20 Hz and dark rough areas are observed around the dark facet.The fatigue data tested at different frequencies corresponds with each other which imply that distinct frequency effect is not found on the ultra-high fatigue properties of Ti-6Al-4V.Reliability fatigue life decreases more when the reliability increases than when confidence level does to the same degree.A new fatigue model (p-γ-S-N curve) is also established to predict the reliability fatigue life.
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